Fender assembly and method for locating a substructure or barge in a moonpool of a ballastable transporter

ABSTRACT

The invention relates to a fender assembly which forms part of a floatable ballastable transporter ( 1 ) comprising pontoons ( 6 ) which define a moonpool ( 4 ) for the location of an offshore platform substructure ( 3 ) or a barge, and a load transfer construction ( 7, 10, 12, 13, 14, 25, 26, 27, 28 ) above the pontoons ( 6 ).  
     The fender assembly comprises fender support structures ( 16 ) on opposite sides of the moonpool ( 4 ), the fender support structures ( 16 ) having sliding supports ( 17 ) sloping towards the moonpool ( 4 ), and fender members ( 20 ) which are slidingly supported by the sliding supports ( 17 ).  
     The invention also relates to a method for locating and centring a substructure ( 3 ) or barge in a moonpool ( 4 ) by using a fender assembly according to the invention.

[0001] The invention relates to a fender assembly which forms part of a floatable ballastable transporter comprising a moonpool for the location of a substructure or a barge, in order to transfer an offshore platform topsides between the transporter and the substructure or barge.

[0002] The invention also relates to a method for locating a substructure or barge in a moonpool of a ballastable transporter.

[0003] An offshore platform consists of a substructure made from steel or concrete and a topsides comprising one or more decks, production equipment and other facilities which are required to exploit a subsea hydrocarbon reserve.

[0004] When developing an offshore hydrocarbon reserve, the substructure and the topsides are normally separately fabricated. The substructure can be installed on the sea bed, and the topsides can then be placed on the substructure. The placing of the topsides on the substructure can be done by first placing the topsides, which is made at a construction yard, on a floatable ballastable transporter. The transporter is then moved to the substructure, the transporter is deballasted to a level in which the topsides is above the substructure, and the substructure is located in the moonpool by moving the transporter into a position in which it embraces the substructure. The transporter is then ballasted, causing the transporter to descend, and the topsides is transferred to the substructure.

[0005] As an alternative to placing the topsides directly on the transporter at the construction yard, the topsides may first be placed on a barge or ship. The barge or ship with the topsides is sailed to the location of the transporter, the barge or ship with the topsides is moved into the moonpool of the transporter with the topsides above the transporter, the transporter is deballasted, and the topsides is transferred to the transporter. Then the topsides is transferred to the substructure as explained above.

[0006] The transporter may also be used for the removal of a topsides from a substructure, and transferring a topsides from the transporter to a barge. Irrespective of which operation is carried out, the main principle is the same: the substructure or barge is located in the moonpool of the transporter, and the topsides is lowered onto or lifted up from the substructure or barge by lowering or raising the transporter by ballasting or deballasting.

[0007] A suitable transporter, for which the fender assembly according to the invention forms a part, comprises pontoons which define a moonpool for the location of a substructure or a barge; a topsides load transfer construction above the pontoons, for transferring load from a topsides above the moonpool to the transporter; and structural elements interconnecting the pontoons and the topsides load transfer construction. Such a transporter is described in WO 99/06270.

[0008] The sea is almost always in motion, and consequently there will almost always be a relative movement between the transporter and the substructure or barge. This relative movement is partly translatory and partly rotational, it is partly a drift movement, caused by ocean streams and wind, and partly a shifting or oscillating movement with various frequencies, caused by waves. Preferably this relative movement should be dampened before the transfer of the topsides is carried out. Further the substructure or barge have to be adequately centred in the moonpool of the transporter, in order to align corresponding load transfer structures of the topsides and the transporter or substructure or barge before a lift-off or setting down of the topsides. The relative movement is very complex, but for the problems related to dampening and centring of the substructure or barge in the moonpool, the movement in surge direction, i.e. the longitudinal direction of the transporter, in which the transporter moves when it embraces the substructure, and the sway direction, i.e. the transverse direction of the transporter, are the most important. The transporter, the substructure and barge are heavy structures, and a large dampening of the relative movement will therefore cause tremendous forces between the transporter and the substructure or barge. If these forces are too big, they will cause damage to the transporter or substructure or barge.

[0009] The object of the invention is to provide a fender assembly for dampening relative movement between the transporter and the substructure or barge in sway direction, in which the forces between the transporter and the substructure or barge do not exceed a predetermined value. Preferably the fender assembly shall also enable centring in sway direction of the substructure or barge in a moonpool of the transporter. A further object is to provide a method for locating and centring a substructure or barge in a moonpool of a ballastable transporter, in which forces between the transporter and the substructure or barge are dampened and do not exceed a predetermined value during relative movement between the transporter and the substructure or barge in sway direction. A further object is that the maximum force between the transporter and the substructure or barge, and also the dampening, can be selected according to operational requirements.

[0010] The objects are achieved with a fender assembly according to claim 1, a method according to claim 8 and a combination of a fender assembly and a substructure or barge according to claim 10.

[0011] The invention thus relates to a fender assembly for dampening relative movement in sway direction between a transporter of the above kind and a substructure or barge located in the moonpool of the transporter. The fender assembly comprises fender support structures which are secured to the topsides load transfer construction on opposite sides of the moonpool, and which have sliding supports sloping towards the moonpool. Further the fender assembly comprises fender members which are slidingly supported by the sliding supports and are movable along the sliding supports between an upper position and a lower position.

[0012] If nothing holds them back, the fender members will due to the force of gravity be located in the lower position. If a substructure or barge is located in the moonpool, and if there is a certain minimum relative movement in sway direction between the transporter and the substructure or barge, the fender members will be pushed up the sliding supports. The forces which push the fender members up the sliding supports are partly counteracted by a friction force in the sliding supports, which causes an energy absorbtion and a dampening of the relative movement between the transporter and the substructure or barge.

[0013] The force which is required to push the fender members up the sliding supports depend upon the weight of the fender members, the angle of the sloping sliding supports and the friction in the sliding supports. All these parameters can be controlled by the actual design of the fender assembly, and the force which is required to push the fender members up the sliding supports can thereby be predetermined. Thus the forces between the transporter and the substructure or barge do not exceed a predetermined value during the relative movement in sway direction between the transporter and the substructure.

[0014] Preferably the fender assembly comprises fender elevators arranged to selectively pull the fender members up the sliding supports or letting the fender members slide down the sliding supports by the force of gravity. This enables pulling the fender members up from their lower position and widening out the distance between fender members located on opposite sides of the moonpool, in order to provide a clearance for a substructure or barge which enters the moonpool.

[0015] The invention also relates to a combination of a fender assembly and a substructure or barge, in which the distance between fender members on opposite sides of the moonpool in the lower position of the fender members is essentially equal to or slightly larger than the width of the substructure or barge. If there is a relative movement in sway direction between a substructure or barge which is located in the moonpool and the transporter, this will cause only one of the fender members to be pushed up the sliding support. This force between the fender member and the substructure or barge will counteract the relative movement, and cause a centring of the substructure or barge in the moonpool.

[0016] The invention also relates to a method for locating a substructure or barge in a moonpool of a ballastable transporter, using a fender assembly comprising fender elevators, the distance between fender members on opposite sides of the moonpool in their lower position being essentially equal to the width of the substructure or barge, which method comprises:

[0017] a) pulling fender members on opposite sides of the moonpool up the sliding supports by activating the fender elevators, to provide adequate clearance for entering the substructure or barge in the moonpool,

[0018] b) entering the substructure or barge in the moonpool by a relative movement between the transporter and the substructure or barge in the surge direction,

[0019] c) letting the fender members slide down the sliding supports by the force of gravity by deactivating or releasing the fender elevators, thereby reducing or eliminating the clearance between the fender members and the substructure or barge, whereupon

[0020] a relative movement between the transporter and the substructure or barge in the sway direction pushes a fender member up a sliding support and produces a centring force between the fender member and substructure or barge.

[0021] Thereby, as explained above, the relative movement between the transporter and the substructure or barge in the sway direction is dampened, and the forces between the transporter and the substructure or barge do not exceed a predetermined value, as discussed above.

[0022] The invention will now be explained in more detail in association with a description of a specific embodiment, and with reference to the drawings, in which:

[0023]FIG. 1 is a perspective view of a transporter with a fender assembly according to the invention,

[0024]FIG. 2 is a side view of the transporter in FIG. 1 in the process of removing a platform topsides from a substructure,

[0025]FIG. 3 is a sectional view defined by the arrows III-III in FIG. 2, illustrating the transporter and a substructure,

[0026]FIG. 4 is a perspective view of a part of the transporter, illustrating the fender assembly according to the invention,

[0027]FIG. 5 is a sectional view defined by the arrows V-V in FIG. 3, illustrating the fender assembly according to the invention,

[0028]FIG. 6 illustrates a fender member and a fender load transfer structure, and

[0029]FIG. 7 illustrates forces between a transporter and a fender member when using the fender assembly according to the invention.

[0030]FIG. 1 is a perspective view of a ballastable transporter 1 comprising lower pontoons 6 which define a moonpool 4 and upper tubulars 7 which are located above and are parallel to the pontoons 6 and define an opening 18 above the moonpool 4. The moonpool 4 and the opening 18 essentially correspond to each other, thereby forming a vertical recess through the transporter 1, which recess is horizontally accessible from a open side 5 of the transporter. The transporter 1 also comprises structural elements which interconnect the pontoons 6 and the tubulars 7. In the illustrated embodiment the structural elements are columns 8 which are perpendicular to the pontoons 6 and the tubulars 7. A support bridge 29 interconnects the tubulars 7.

[0031] For ballasting purposes the pontoons 6 have ballasting chambers. Additionally the structural elements, i.e. the columns 8, and the tubulars 7 may also comprise ballasting chambers.

[0032] The transporter 1 floats in the sea, with the pontoons 6 down and the columns 8 vertical. References to “upper”, “lower”, “above”, “below”, “vertical” etc. in this patent application refers to the way the transporter floats in the sea.

[0033] Further, not illustrated, the transporter comprises piping, valves, pumps with motors and control equipment for performing the ballasting/deballasting. The transporter may be manned or unmanned, it may be moved by tugs or have its own propulsion machinery.

[0034]FIG. 2 is a side view of the transporter 1 in the process of removing a platform topsides 2 from a substructure 3. The illustrated substructure 3 is a jacket, i.e. a steel trusswork, which rests on the seabed 15. The sea surface is identified by reference numeral 19. The substructure 3 and the topsides 2, which is a steel construction comprising one or more decks and equipment which is necessary for the intended purpose, together form an offshore platform.

[0035] In FIG. 2 the transporter 1 is floating besides the substructure 3, with the substructure located in the vertical recess formed by the moonpool 4 and the opening 18. This has been achieved by moving the transporter 1 with the open side 5 towards the substructure 3, until the substructure is located in the moonpool 4.

[0036] Before moving the transporter 1, the draft of the transporter was adjusted by ballasting, to bring the elevation of support arms 10 for supporting the topsides 2 below the elevation of the underside of the topsides 2.

[0037] The transporter 1 is then deballasted, which leads to a reduced draft and a lifting (not illustrated) of the topsides 2, up from the substructure 3. The transporter 1 with the topsides 2 is then free to move away (not illustrated) from the substructure 3. The removal of the topsides 2 from the substructure 3 is then completed.

[0038]FIG. 4 is a perspective view of a part of the transporter, illustrating a tubular 7 and a support arm 10 for the topsides. Both FIG. 1 and FIG. 4 also illustrates a support beam 28, which is located above the moonpool 4, and which is supported by struts 9 extending from the pontoon 6 (struts 9 are not illustrated in FIG. 4). The support beams 28 are tied to the tubulars 7 by tie-backs 12. The support arm 10 rests in one end on a chair 13 on the support beam 28, and is in its other end secured to the tubular 7 by a support frame 14. The support arm 10 is further provided with two link arms 27 which are movable in the horizontal plane by hinges 26. The ends of the link arms 27 are provided with dowels 25 for mating with corresponding lifting brackets on the underside of the topsides. The items designated by reference numerals 7, 10, 12, 13, 14, 25, 26, 27 and 28 form part of a topsides load transfer construction, and may have various designs.

[0039] Further FIG. 4 illustrates two fender support structures 16 which are secured to the support beam 28, and on their upper side have sliding supports 17 sloping towards the moonpool 4. Fender members 20 are slidingly supported by the sliding supports 17 and are movable along the sliding supports 17.

[0040]FIG. 5 is a sectional view defined by the arrows V-V in FIG. 3, illustrating the support beam 28, the tie-back 12, the strut 9 and the fender support structure 16 with the sliding support 17. The fender member is illustrated with full lines in a lower position 20, and in dotted lines in an upper position 20′. In its lower position 20, the fender member abuts a structural member of the substructure 3.

[0041]FIG. 3 is a sectional view defined by the arrows III-III in FIG. 2, illustrating the transporter 1 and the substructure 3 located in the moonpool 4. FIG. 3 also illustrates the fender support structures 16, and it is seen that fender support structures 16 are located on opposite sides of the moonpool 4. The fender members are illustrated in their upper position 20′. It is seen that the substructure 3 has a width w across the moonpool 4, and it is further seen that in the upper position 20′ of the fender members, the distance b between fender members on opposite sides of the moonpool 4 is larger than the width w of the substructure 3. The fender members have oblique or tapering entrance portions 36 facing the open side 5, to guide the transporter around the substructure during the entering of the substructure 3 in the moonpool 4.

[0042] As discussed in the general part of the description, the sea is almost always in motion, which means that there will be some relative movement between the transporter 1 and the substructure 3, which might cause collisions and damage both to the transporter and the substructure. If this relative movement is not dampened before the topsides is lifted off the substructure, the topsides may also be damaged. Further the substructure have to be adequately centred in the moonpool of the transporter, in order to align the dowels 25 of the transporter with the lifting brackets on the underside of the topsides before the topsides is lifted off the substructure.

[0043] As mentioned, the relative movement is very complex, but for the problems related to dampening and centring of the substructure in the moonpool, the movement in surge direction x (se FIG. 3), i.e. the longitudinal direction of the transporter, in which the transporter moves when it embraces the substructure, and the sway direction y, i.e. the transverse direction of the transporter, are the most important.

[0044] Still with reference to FIG. 3, in their lower position 20, the distance b between the fender members on opposite sides of the moonpool 4 is essentially equal to the width w of the substructure 3. Due to the force of gravity, the fender members will when they are allowed to slide freely down the sliding support 17, move towards their lower position 20. If there is a relative movement in sway direction y between the substructure 3 and the transporter 1, this will cause the fender member on one of the sides of the moonpool 4 to be pushed up its sliding support 17, which will create a force between the fender member and the substructure, which force counteracts the relative movement. Since the distance b between the fender members on opposite sides of the moonpool 4 is essentially equal to the width w of the substructure 3, the relative movement between the substructure and the transporter in sway direction y will create a gap between the substructure 3 and the other fender member, and the other fender member will therefore rest in its lower position 20. Thus there will be a force between the substructure and the fender only on that side of the substructure which is most distant from the centre of the moonpool 4, and this will cause a centring of the substructure 3 in the moonpool 4 in sway direction y.

[0045] Preferably the distance b between the fender members on opposite sides of the moonpool in their lower position 20 is slightly larger than the width w of the substructure 3, in order to facilitate the entering of the substructure in the moonpool and ensure a suitable gap between the substructure and the fender on one of the sides of the substructure when there is a relative movement between the substructure and the transporter.

[0046] A typical jacket may have a width w of 30 metres. For such a jacket the distance b between the fender members on opposite sides of the moonpool in their lower position 20 should the be between 29 and 32 metres, preferably between 30 and 31 metres and preferably approximately 30.5 metres, i.e. there will be a clearance of 0.5 metres between the jacket and the fender members when the fender members are in their lower position 20. The actual preferred clearance will have to be determined from operational parameters, such as jacket width and wave motions.

[0047] Both FIG. 4 and FIG. 5 illustrate fender elevators formed by winches 21 which are secured to the support beams 28, and cables 22 extending from the winches 21 to the fender members 20. These fender elevators enables pulling the fender members up the sliding supports. A detaching or release of the fender elevators, which can be done by letting out cable from the winches, causes the fender members to slide down the sliding supports 17 due to the force of gravity, which in FIG. 5 is illustrated by the arrow g. In this way the fender elevators 21 can selectively pull the fender members 20 up the sliding supports 17 or letting the fender members 20 slide down the sliding supports 17 to any position between the lower position 20 and the upper position 20′.

[0048] The fender members may have different shapes, and may be purpose designed for contact points of the substructure. Preferably, however, as illustrated in FIG. 3, the fender members 20 are elongated and extend along the sides of the moonpool 4. This enables using the same fender members for various substructures, and also makes the positioning of the substructure in the moonpool less critical than if the fender members were purpose designed for the contact points. Further, an elongated, continues fender member will guide the transporter when embracing the substructure, and thus facilitate the entering of the substructure in the moonpool.

[0049] The fender members can be made from different types of elements, e.g. different types of steel profiles which can withstand the forces. FIG. 6 illustrates a preferred fender member 20 and a fender support structure 16. The fender member is made from steel pipe, and is provided with a recess 34 with two rail brackets 31. The fender support structure 16 is a welded steel plate construction with a flange 33 which is enclosed by the rail brackets 31, thereby forming the sliding support 17. Low friction liners 32 are located between the wall of the fender member 20 and the flange 33 and between the rail brackets 31 and the flange 33, to ensure a predictable friction between the fender member 20 and the fender support structure 16. The fender support structure also have a flange 35 which is welded to the support beam 28 (not illustrated in FIG. 6). Not illustrated stops prevent the fender member from sliding beyond its upper and lower positions.

[0050] Preferably fender boards 24 are arranged on the side of the fender members 20 facing the substructure 3, as illustrated in FIGS. 4 and 5. The fender boards 24 may be made of a material which is softer than steel, e.g. a laminate of rubber and thin steel plates, and may be attached to the fender members by not illustrated bolts. The relatively soft fender boards 24 will ensure that the initial contact between the fender member 20 and the substructure 3 does not cause any damage.

[0051] When locating and centring a substructure 3 in a moonpool 4 of a ballastable transporter 1 by the method according to the invention, a fender assembly comprising fender elevators is used, and the distance b between the fender members on opposite sides of the moonpool in their lower position 20 is essentially equal to the width w of the substructure 3. The following steps are carried out:

[0052] a) pulling fender members 20 on opposite sides of the moonpool 4 up the sliding supports 17 by activating the fender elevators 21, to provide adequate clearance for entering the substructure 3 in the moonpool,

[0053] b) entering the substructure 3 in the moonpool 4 by a relative movement between the transporter 1 and the substructure 3 in the surge direction x,

[0054] c) letting the fender members 20 slide down the sliding supports 17 by the force of gravity g by deactivating or releasing the fender elevators 21, thereby reducing or eliminating the clearance between the fender members and the substructure 3, whereupon

[0055] a relative movement between the transporter and the substructure 3 in the sway direction y pushes a fender member 20 up a sliding support 17 and produces a centring force L between the fender member 20 and substructure 3.

[0056] The fender members 20 are preferably adapted to carry ballast. Such ballast can be made from steel or concrete. An easy available ballast material which is easy to handle is, however, water. The fender members 20 therefore preferably have ballast compartments 23 for ballast water, as illustrated in FIG. 4, which are formed by partition walls 30 which are welded into the fender member pipe 20. In addition, not illustrated, there are valves, pumps and piping or hoses etc. for letting water in an out of the ballast water compartments 23.

[0057] By adjusting the ballast of the fender members 20 they can be given a predetermined weight. The forces which are required to move the fender members 20 up the sliding supports 17, i.e. the forces between the fender members and the substructure, depend upon the sloping angle of the sliding supports, the friction in the sliding supports 17 and the weight of the fender members 20. A steeper angle of the sliding support, a greater friction in the sliding supports and a bigger weight of the fender members all cause an increase in the required forces to move the fender members up the sliding supports. The ballasting of the fender members is a convenient way of adjusting these forces.

[0058] The advantages of the invention will now be explained with reference to FIG. 7, which applies to the situation in FIGS. 2 and 3, i.e. a stationary substructure is located in the moonpool of a transporter which is moving in the sea. The transporter has a fender assembly according to the invention, comprising ballastable fender members with fender boards made from elastic rubber. FIG. 7 illustrates forces L between each fender member and a contact point of a leg of the substructure as a function of displacement D of the transporter. L has the unit tonnes, while the displacement D of the transporter has the unit metres. The spring constant for the fender board is for illustrative purposes assumed to be linear until it is completely compressed, and then totally stiff. The steel pipe of the fender member is assumed to be totally stiff.

[0059] With reference to FIG. 3 it is understood that the forces L and the displacement D acts in the sway direction y.

[0060] Case for a Non-ballasted Fender Member (Graph A)

[0061] Initially the fender member is in its lower position, and there are no contact between the fender member and the substructure, i.e. both L and D are zero (point I).

[0062] The transporter then moves towards the substructure, and the fender member contacts the substructure, which results in a compression of the fender board and a gradually increase in the force L. Due to the linear spring constant of the fender board, the force L increases in proportion to the compression D of the fender board, until the fender board is fully compressed with an amount x₁ (point II).

[0063] A further movement of the transporter towards the substructure requires that the fender member is pushed up the sliding support from its lower position. To achieve this, the force L must overcome the sum of a force which is required to raise the level of the fender member, which is a function of the weight of the fender member and the angle of the sliding support, and a friction force which acts between the fender member and the fender support structure in the sliding support, and which is a function of the friction constant, the weight of fender member and the angle of the sliding support. This means that the force L increases until F₂, for which both these forces are overcome (point III).

[0064] Now a further movement of the transporter towards the substructure will push the fender member up the sliding support. As long as the fender member is within its sliding range x₂, i.e. the distance between the lower and the upper position of the fender member, both the force which is required to raise the elevation of the fender member and the friction force are constant, and hence L is constant F₂. The movement of the fender member continues until the movement of the transporter stops (point IV).

[0065] Due to wave induced forces, the transporter then starts its movement in the opposite direction, and the fender member starts sliding down the sliding support, but is prevented from free sliding by the force L from the substructure. The friction force will always act opposite the direction of movement, and the force L therefore drops from F₂ to F₁ when the direction of the movement of the transporter changes (point V).

[0066] The fender member slides down the sliding support while the force L is constant F₁, until the fender member reaches its lower position (point VI).

[0067] The force L then drops until the level in which the fender board starts decompressing (point II), and the fender board is linearly decompressed until the force L is zero (point I).

[0068] Thus, in the compression of the fender board from point I to II, energy is accumulated by the fender board. There is, however, no friction force which is overcome during the compression of the fender board, and consequently the fender board absorbs no energy. In the displacement of the fender member from III to IV, and in the return displacement from V to VI, there is a friction force which causes energy absorbtion, and consequently dampening of the transporter's movement.

[0069] With reference to FIG. 3 it is understood that it is essentially the movement in the sway direction y which is dampened by the a fender assembly according to the invention. Movements in other directions will partly be dampened by friction between the fender boards and the substructure, and partly by other not illustrated dampening means which do not form part of the invention.

[0070] The maximum force F₂ depend upon the friction constant in the sliding support, which can be selected by an appropriate material in the low friction liners; the angle of the sliding support, which is a design choice; and the weight of the fender member. All these parameters are known or predictable, and the maximum force F₂ between the transporter and the substructure can thereby be predetermined by calculations according to the laws of mechanics.

[0071] Case for a Fully Ballasted Fender Member (Graph B)

[0072] Graph B illustrates the case for a fully ballasted fender member.

[0073] The transporter's movement towards the substructure and the compression of the fender board is as for the non-ballasted case, i.e. point I and II applies also to graph B.

[0074] The ballasted fender member is heavier than the non-ballasted fender member, and consequently the force L which is required to push the fender member up the sliding support is bigger. Thus, no movement of the fender member takes place until the force L is increased to F₄ (point VII).

[0075] The fender member will then slide along the sliding support while the force L is constant F₄, until the movement of the transporter stops (point VIII).

[0076] The transporter then starts its movement in the opposite direction, and the force L drops from F₄ to F₃ (point IX).

[0077] The fender member slides down the sliding support while the force L is constant F₃, until the fender member reaches its lower position (point X).

[0078] The force L then drops until the level in which the fender board starts decompressing (point II), and the fender board is linearly decompressed until the force L is zero (point I).

[0079] Compared to the non-ballasted fender member, the ballasting of the fender member increases the maximum force between the transporter and the substructure to F₄. Since the dampening is proportional to the friction force, and the friction force is a function of the weight of the fender member, the ballasting of the fender member also increases the dampening.

[0080] Thus, by appropriate ballasting, within the range provided by the possible ballasting, both the maximum force between the transporter and the substructure and the dampening can be selected according to operational requirements.

[0081] It should be understood that the above considerations is equally applicable for an installation of a topsides which is located on a ballastable transporter, wherein a substructure is located in the moonpool of the transporter, and the transporter is ballasted until the topsides is lowered onto the substructure. It should also be understood that similar considerations will apply when a barge or any other structure is located in the moonpool of the transporter instead of the substructure. 

1. A fender assembly which forms part of a floatable ballastable transporter (1), the ballastable transporter (1) comprising: pontoons (6) which define a moonpool (4) for the location of a substructure (3) or a barge; a topsides load transfer construction (7, 10, 12, 13, 14, 25, 26, 27, 28) above the pontoons (6), for transferring load from a topsides (2) above the moonpool (4) to the transporter (1); structural elements (8, 9) interconnecting the pontoons (6) and the topsides load transfer construction (7, 28); the topsides (2) can be lowered onto or lifted up from a substructure (3) or barge located in the moonpool (4) by lowering or raising the transporter (1) by ballasting or deballasting; the fender assembly dampen relative movements in the sway direction (y) between the transporter (1) and the substructure (3) or barge, the fender assembly is characterized by fender support structures (16) secured to the topsides load transfer construction (28) on opposite sides of the moonpool (4), the fender support structures (16) having sliding supports (17) sloping towards the moonpool (4), fender members (20) which are slidingly supported by the sliding supports (17) and are movable along the sliding supports (17) between an upper position (20′) and a lower position (20), and fender elevators (21) arranged to selectively pull the fender members (20) up the sliding supports (17) or letting the fender members (20) slide down the sliding supports (17) by the force of gravity (g).
 2. A fender assembly according to claim 1, wherein the fender elevators comprise winches (21) secured to the topsides load transfer construction (28) and cables (22) extending from the winches (21) to the fender members (20).
 3. A fender assembly according to claim 1 or 2, wherein the fender members (20) are adapted to carry ballast.
 4. A fender assembly according to any of the preceding claims, wherein the fender members (20) have ballast compartments (23) for ballast water.
 5. A fender assembly according to any of the preceding claims, wherein the fender members (20) are elongated and extend along the sides of the moonpool (4).
 6. A fender assembly according to any of the preceding claims, wherein fender boards (24) are arranged on the side of the fender members (20) facing the substructure (3) or barge.
 7. A method for locating and centring a substructure (3) or barge in a moonpool (4) of a ballastable transporter (1) as defined in the preamble of claim 1, using a fender assembly according to any of the claims 2-7, the distance (b) between the fender members on opposite sides of the moonpool in their lower position (20) being essentially equal to the width (w) of the substructure (3) or barge, characterized by: a) pulling fender members (20) on opposite sides of the moonpool (4) up the sliding supports (17) by activating the fender elevators (21), to provide adequate clearance for entering the substructure (3) or barge in the moonpool, b) entering the substructure (3) or barge in the moonpool (4) by a relative movement between the transporter (1) and the substructure (3) or barge in the surge direction (x), c) letting the fender members (20) slide down the sliding supports (17) by the force of gravity (g) by deactivating or releasing the fender elevators (21), thereby reducing or eliminating the clearance between the fender members and the substructure (3) or barge, whereupon a relative movement between the transporter and the substructure (3) or barge in the sway direction (y) pushes a fender member (20) up a sliding support (17) and produces a centring force (L) between the fender member (20) and substructure (3) or barge.
 8. A method according to claim 7, using a fender assembly according to any of the claims 3-6, wherein the fender members (20) are ballasted to a predetermined weight.
 9. A combination of a fender assembly according to any of the claims 1-6 and a substructure (3) or barge, characterized by the distance (b) between fender members on opposite sides of the moonpool in their lower position (20) being essentially equal to the width (w) of the substructure (3) or barge.
 10. A combination of a fender assembly and a substructure (3) or barge according to claim 10, wherein the distance (b) between the fender members on opposite sides of the moonpool in their lower position (20) is slightly larger than the width (w) of the substructure (3) or barge. 